A major police manhunt is under way after a one-year-old boy died and a girl was left fighting for life following an attack at a flat in Finsbury Park.
Murder detectives, who have seized a hammer found nearby, are searching for Bidhya Sagar Das, 33.
Mr Das lives at the apartment in Wilberforce Road where the injured children were found.
He is believed to be the child's father and members of the public have been urged to dial 999 immediately if they see him.
A hammer reportedly found dumped in a bag of sand near the apartment is being examined by forensics officers, although the Met would not comment on its relevance in the investigation.
Scotland Yard said officers were alerted to the incident at 11.10pm on Saturday.
Police said Mr Das left the flat shortly before the emergency services were called and has not been seen since.
Neighbours said a woman was heard frantically shouting for help before scores of police arrived at the scene.
Medics rushed the injured children, believed to be twins, to the Royal London Hospital.
One child was taken to the hospital by air ambulance and the other by ambulance.
The boy was pronounced dead shortly before 1am in hospital, while the girl remains there in a critical condition.
Detective Chief Inspector Dave Whellams told Sky News police had received a call from a "distraught" member of the public.
He added that the young girl is being operated on.
A Met Police spokesman said: "Detectives are urgently trying to trace a man who was at the flat shortly before the injured children were found, but left before emergency services were called. The man has not been seen since this time.
"If any members of the public see this man or know his whereabouts, they are urged to contact police via 999 immediately."
Witnesses heard the woman shouting for help on the street outside and later saw two children being carried out of the property.
Mihai Manea, 29, who lives on the second floor of the building, said the children were twins and lived on the top floor with their Romanian mother and Indian father.
The father worked at the nearby Pembury Hotel as a receptionist, where Mr Manea also works, until a few days ago, he said.
A woman living opposite the building, who gave her name as Gui Gui, said she heard a woman shouting late last night and opened the window to offer help.
"I was watching TV," she said. "I heard someone was shouting. She kept on shouting. I do not know what she was shouting.
"I opened the window and I asked her 'Can I help you, can I call the police for you?' She said, 'My kids'."
She later saw two young children being carried out of the building, with one being held very close to a member of the emergency services.
Mr Manea described the mood inside the property as "tense".
"It's really tense," he said. "There's police coming and going all the time."
On Sunday morning, police cars were parked outside a white three-storey building where the children were injured.
Police officers were visiting and searching properties along the tree-lined street.
Detective Chief Inspector Dave Whellams, of the Homicide and Major Crime Command, said: "This is clearly a tragic incident, and our thoughts are with the family of the two young children.
"Despite the best efforts of medical professionals a baby boy sadly died in the early hours of this morning. A baby girl currently remains in a critical condition and is receiving specialist medical care.
"We are in the early stages of the investigation and our inquiries have led us identify Bidhya Sagar Das, who we believe was at the flat shortly before the injured children were found late on Saturday night.
"It is of paramount importance that he is located immediately and we are urgently seeking the public's help in the hope that they will be our eyes and ears and provide us with any useful information that would assist with tracing him.
"As the hours pass the need to locate Bidhya Sagar Das becomes more pressing.
"He may have approached friends or family in the aftermath of the incident and I am specifically appealing to these people to come forward. "
The Met added: "Next of kin are aware. Formal identification awaits and a post-mortem examination will be scheduled in due course."
The force urged members of the public, who have been warned not to approach Das, to contact 999 if they see him or know of his whereabouts.
Anyone with information that may assist the investigation is asked to contact the incident room on 020 8345 3775 or Crimestoppers on 0800 555 111.